1. Field of the Invention
This invention relates to a steel material having on its surface a multilayer protective coating which makes it resistance to wear and corrosion.
2. Description of the Prior Art
A known multilayer-coated corrosion-resistant steel material is shown by way of example in FIG. 3. It comprises, for example, a steel sheet 11 having a galvanised layer 12 formed on its surface 11', a chromate film 13 on the galvanised layer 12 and a resin layer 14 on the chromate film 13.
Although zinc is often used to provide a protective coating on steel, the zinc layer must be of considerable thickness in order to protect the steel from corrosion effectively, since the protection of steel by means of a galvanised layer results from sacrificial corrosion of the zinc.
The galvanised layer 12 must have a thickness of at least about 30 microns in view of the severe natural conditions to which the coated material will be exposed when it is, for example, to be used for making motor vehicle parts. The formation of such a thick layer is time-consuming which brings about a drastic reduction in productivity. As the galvanised layer 12 is so thick, it tends to crack or peel away when the coated material is pressed or bent into a desired shape. Moreover, the resin layer 14 tends to have pinholes and is also liable to crack when the coated material is pressed or bent. The corrosion product of the zinc tends to accumulate quickly in the pinholes or cracks of the resin layer 14 despite the presence of the chromate film 13. As a result, the resin layer 14 tends to peel away as the chromate film 13 loses it effectiveness, particularly when the coated material is used to make, for example, motor vehicle parts and is exposed to highly corrosive conditions including large temperature variations.